;;
;; Copyright (c) 2010, Justin Grant <justin at imagine27 dot com>
;; All rights reserved.

;; Redistribution and use in source and binary forms, with or without modification
;; are permitted provided that the following conditions are met:

;; Redistributions of source code must retain the above copyright notice, this list
;; of conditions and the following disclaimer.
;; Redistributions in binary form must reproduce the above copyright notice, this
;; list of conditions and the following disclaimer in the documentation and/or
;; other materials provided with the distribution.
;; Neither the name of the <ORGANIZATION> nor the names of its contributors may be
;; used to endorse or promote products derived from this software without specific
;; prior written permission.
;;
;; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
;; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
;; WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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;;



(defn merge- [left right]
  (cond (nil? left) right
        (nil? right) left
        true (let [[l & *left] left
                   [r & *right] right]
               (if (<= l r) (cons l (merge- *left right))
                   (cons r (merge- left *right))))))

(defn merge-sort [lst]
  "Implements the well-known merge-sort sorting algorithm using lists
   O(n log n) time, O(n) space"
  (let [[l & *lst] lst]
    (if (nil? *lst)
      lst
      (let [[left right] (split-at (/ (count lst) 2) lst)]
        (merge- (merge-sort left) (merge-sort right))))))


(println (merge-sort '(3 2 5 9 1 2 6)))
;;(1 2 2 3 5 6 9)
